All I can think of is the word idol. Is it the most correct answer to this question? Or is there any more appropriate word?
EDIT:
In my opinion, idol sounds a bit too "strong".
When someone is a fan of another person, I think it usually means he likes the other one and probably follows his activities etc.
When the other person is his idol, I think it means he idolizes the other person and maybe tries to do everything like him, wear same clothes, etc.
Just my thought..
Example: Uncle John is my girlfriend's _. She likes him a lot.
Best Answer
In crossword puzzles, it's usually "idol".
Various words suggest various degrees. It might be as mild as "a preference". An "object of interest" or an "object of devotion".
So it's someone you like? or someone you follow (as on Twitter)? Maybe you should just say that, instead of saying you're a "fan".